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Molecular Mechanisms by Which Salvinorin a Binds to and Activates the Kappa-Opioid Receptor

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Molecular Mechanisms by Which Salvinorin a Binds to and Activates the Kappa-Opioid Receptor MOLECULAR MECHANISMS BY WHICH SALVINORIN A BINDS TO AND ACTIVATES THE KAPPA-OPIOID RECEPTOR by FENG YAN Dissertation Adviser: Bryan L. Roth, M.D./Ph.D. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Biochemistry CASE WESTERN RESERVE UNIVERSITY May, 2008 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the dissertation of Feng Yan ______________________________________________________ candidate for the Ph.D. degree *. (signed) Vernon E. Anderson, Ph.D. ______________________________________________ (chair of the committee) Bryan L. Roth, M.D., Ph.D. ______________________________________________ Martin D. Snider, Ph.D. ______________________________________________ Paul R. Ernsberger, Ph.D. ______________________________________________ Krzysztof Palczewski, Ph.D. ______________________________________________ ______________________________________________ May, 2008 (date) ________________________ * We also certify that written approval has been obtained for any proprietary material contained therein. TABLE OF CONTENTS List of Tables vi List of Figures viii List of Abbreviations ix Acknowledgements xiv Abstract xvi CHAPTER 1: Introduction 1.1 G Protein-Coupled Receptors………………………………………..……………..…1 1.1.1 G Protein-Coupled Receptor Overview…………………………………………1 1.1.2 G Protein-Coupled Receptor Structures………………………………………...1 1.1.3 G Protein-Coupled Receptor Function……………………..…………………...3 1.2 Opioid Receptors…………………………………………….………………………..6 1.2.1 Opioid Receptor Overview……………………………………………………...6 1.2.2 Opioid Receptor Function………………………………………………………8 1.2.3 Opioid Receptor Signaling Regulation………………………………………...11 1.3 Opioids ………………………………………………………………………………11 1.3.1 Endogenous Opioids…………………………..……………………………….11 1.3.2 Exogenous Opioids………………………..…………………………………...13 1.3.2.1 MOR ligands……..……………………………………………..………..18 1.3.2.2 DOR ligands ……..……………………………………………….…..…18 1.3.2.3 KOR ligands ……..……………………………………………….……..19 i 1.3.3 Opioids’ Medical Use and Abuse…………………….…………….................20 1.4 The History of Salvia Divinorum and Salvinorin A Research……………………….22 CHAPTER 2: Materials and Methods 2.1 Materials……………………………………………………………………………..27 2.1.1 Chemicals………………………………………………………………………27 2.1.1.1 Commercial chemicals……………………..…………………………….27 2.1.1.2 Synthesized salvinorin A derivatives……………...…………….………27 2.1.2 cDNA constructs……………………………………………………………….28 2.1.2.1 Sub-cloning of hKOR into pUniversal-signal…………………………...28 2.1.2.2 Generation of KOR mutants by site-directed mutagenesis……………....28 2.1.3 Cells…………………………………………………………………………....28 2.1.3.1 Transient transfection and cell membrane preparation..…………………28 2.1.3.2 Stable expression of G proteins………………………………………….29 2.2 Methods……………….……………………………………………………………..29 2.2.1 Radioligand Binding Assays……………………………………………….......30 2.2.1.1 Competition binding assay………………………………………..……...30 2.2.1.2 Saturation binding assay …………………………………………..…….30 2.2.2 SCAM……………………………………………………………………….....31 2.2.2.1 MTSEA reaction…………………………………………………………31 2.2.2.2 SCAM radioligand binding assay………………………………………..32 2.2.2.3 Determination of second order rate constants…………………………..33 2.2.2.4 Ligand protection against the MTSEA reaction…………………………33 ii 2.2.3 Functional Assays..………………………………………………………….....34 2.2.3.1 Intracellular calcium mobilization (Ca2+ flux)…………………..……….34 2.2.3.2 [35S]GTPS binding assay..……………………………………………….35 2.2.4 Computer Modeling……………………………………………………………35 2.2.4.1 KOR modeling for binding site………………………………………….36 2.2.4.2 KOR modeling for SCAM……………………………………………….36 2.2.5 Large Quantity of KOR Expression and Purification………………………….39 2.2.5.1 KOR expression………………………………………………………….39 2.2.5.2 Labeling KOR with RB-64 or RB-48……………………………………39 2.2.5.3 Anti-FLAG M2 affinity purification……….……………………………40 2.2.5.4 Ni2+ affinity purification of KOR………………………….…………….40 2.2.5.5 SDS-PAGE and western blot analysis…………………………………..41 2.2.5.6 SDS-PAGE and Coomassie staining…………………………………….41 2.2.6 Mass Spectrometric Analysis…………………………………………………..42 2.2.7 Prepulse Inhibition Animal Study…………………..………………………….43 CHAPTER 3: The Binding Site of Salvinorin A 3.1 Introduction and Rationale…………………………………………….……………46 3.2 Results…………………………..…………………………………………………..50 3.2.1 Receptor-based Binding Site Study …………………………………………...50 3.2.1.1 Key residues in the binding site were identified by a combined mutagenesis/computer-modeling method………………………………..50 iii 3.2.1.2 Specific ligand-receptor interactions determine the orientation of salvinorin A in the binding site…………………………………………..54 3.2.1.3 Molecular modeling to visualize and predict the binding site…………...55 3.2.2 Ligand-based Binding Site Study……………………………………………...57 3.3 Discussion……………………………………………………………………………68 CHAPTER 4: The Conformational Changes of KOR Induced by G Protein Coupling 4.1 Introduction and Rationale…………………………………………………………...74 4.2 Results ……………………………………………………………………………….78 4.2.1 Examine the Conformational Changes of KOR by SCAM……………………78 4.2.1.1 Characterize Cys mutants in TMs 6 and 7 and EL2 of KOR……………78 4.2.1.2 SCAM elucidates conformational changes induced by G proteins……...79 4.2.2 Kinetics Studies Confirmed that Conformational Changes Occur in KOR……84 4.2.3 Conformational changes induced by G protein-coupling have significant effects on agonist affinities…………………………………………………………….86 4.2.4 Refining the Binding Site of Salvinorin A …………………………………….87 4.3 Discussion……………………………………………………………………………90 CHAPTER 5: The Design and Application of Covalently-Bound Agonist to Probe KOR 5.1 Introduction and Rationale…………………………………………………………...95 5.2 Results…………………………..…………………………………………………..100 iv 5.2.1 Characterizing Covalently-Bound Ligands by Molecular Pharmacology Methods………………………………………………………………………100 5.2.1.1 RB-64 and RB-48 are potent KOR agonists……………………………100 5.2.1.2 Covalent labeling WT KOR or mutants with RB-64…………………...101 5.2.2 Characterizing Covalently-Bound Ligands by Mass Spectrometry………….104 5.2.2.1 KOR expression, labeling and purification……………………………..104 5.2.2.2 Mass spectrometric analysis……………………………………………105 5.2.3 In vivo PPI Study of RB-64 ……………………...…………………..….…...111 5.3 Discussion…………………………………………………………………………..115 CHAPTER 6: Future Directions and Conclusions 6.1 Future Directions…………………………………………………………………...119 6.1.1 Quantifying G protein Expression……………………………………………119 6.1.2 Chasing the Active and Global Conformation of KOR……………………...119 6.1.3 Crystallographic Study of KOR………………..………………………..……122 6.2 General Conclusions..………………………………………………………………123 Bibliography………………………………………………………………………...124 v LIST OF TABLES 1.1 Endogenous opioid peptides ………………………………………………………...12 1.2 Select list of endogenous opioid peptides and morphine derivatives…………...…...14 1.3 Pharmacological effects of salvinorin A in select animal studies……………………25 3.1 Key residues in salvinorin A’s binding site …………………………………………48 3.2 Affinity (Ki, nM) of salvinorin A, U69593 and dynorphin A (1-13) binding to the WT KOR and mutants transiently expressed in HEK293 T cells……………..….………51 3.3 Agonist potency (EC50, nM) and relative agonist efficacy (normalized Emax) of salvinorin A, U69593 and dynorphin A (1-13) for the WT KOR and mutants transiently expressed in HEK 293 cells ………………………………………......…53 3.4 Effect of Cys-substitution mutations on salvinorin A and 2-thiosalvinorin B binding to KORs ………………………………………………………………………...…...55 3.5 Residues within specified heavy atom (O, N, etc.) distances between the salvinorin A ligand and the KOR receptor for the proposed binding mode ……………………....56 3.6 Salvinorin A derivatives tested in Dr. Bryan Roth’s lab …………………...……….59 4.1 The Comparison of Common Biochemical and Biophysical Methods Used for GPCR Conformation Study………………………………………………………………….77 3 4.2 Kd and Bmax values of [ H]diprenorphine binding to the WT KOR and EL2 cysteine mutants………………………………………………………………………..….…..79 4.3 Changes in inhibition upon the coupling of G proteins Gα16 and Gαi2………………81 4.4 Second-order rate constants (k, M-1s-1) of MTSEA reaction with Cys mutants of KOR………………………………………………………………………………….85 vi 4.5 Differential effects of G protein overexpression on agonists (salvinorin A, U69593 and Dynorphin A (1-13)) binding affinities……………………………………….…86 4.6 Affinity (Ki, nM) of salvinorin A binding to WT KOR and mutants………..……….87 5.1 The pharmacological profile of RB-64 and RB-48…………………...…………….101 5.2 Mascot searches of chymotryptic peptides from KOR (47 kDa) and KOR (52 kDa)…….…………………………………………………………………………...106 5.3 Detected KOR peptides from chymotryptic digestion by AB 4800 MALDI TOF/TOF…………………………………………………………………………...107 5.4 RB-64 modified peptides predicted by in silico chymotrypsin digestion ……….…110 5.5 Null activity levels (mAmp displacement) in mice treated with vehicle, salvinorin A, or RB-64…………………………………………………………………………….111 vii LIST OF FIGURES 1.1 GPCR signaling……………………………………………………………………….5 1.2 GPCR signaling regulation based on β2AR model..…………………………………6 1.3 Opioid receptors regulate the dopaminergic activity in VTA through the disinhibition mechanism…………………………………………………………………………...10 1.4 Chemical backbones of morphine-derived molecules.………………………………14 2.1 Nucleophilic reaction between MTSEA and Cys side chain –SH (S-)………………32 3.1 The pharmacophore of salvinorin A…………………………………………………47 3.2 Modeling salvinorin A-KOR interactions……………………………………………57 4.1 Gα subunit overexpression was used to stabilize receptor conformations in the various active states………………………………………………………………….76 4.2 SCAM pattern ……………………………………………………………………….83 4.3 KOR model based
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